The new Agora ("beveled bottom") version deprecates mw-ui-primary.
* Use mw-ui-constructive (green) for Login, Create account, and Reset
password.
* Use mw-ui-progressive (blue) for Join <wiki> and Search.
A separate change I90954ea will implement right-aligned and quieter button layout.
Bug: 60596
Change-Id: Ia1fe557e7ebeac011c5da7297e848c1bfee7910f
All of these also use other mw.ui features (e.g. mw-ui-vform,
mw-ui-input, mw-ui-checkbox-label, etc.), so they can't only depend
on mediawiki.ui.button.
Change-Id: I6906533753185f7de42552cff38a86fb0171d1d8
